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Gravity is a force that is exerted between to objects with mass. This force is also what keeps planets, asteroids, comets, and star in orbit. Gravity is the most well known and most mysterious force in the universe. In general we know how it works and can even calculate most of its effects. However there are still questions that remain about its true nature.
Our first understanding of gravity comes from Galileo and Newton. Galileo was one of the first people to observe that all objects fall at the same rate with the only difference being caused by air resistance. This was on of the first hints at a gravitational constant. Newton perfected our understanding of the force creating a universal theory of gravitation. This theory hypothesized that the same force that made objects fall and kept objects firmly on earth also kept the planets and other celestial objects in their orbit.
The basic strength of a gravitational field is determined by two factors. The first is the mass of the object. The more matter the object has the greater its individual gravitational field. The next factor is the distance between two objects interacting. Two objects close to one another will have exert a strong pull than those far apart.
Einstein gave us a new understanding of gravity. He proposed that gravity was not just a force but actually a curve in space time. This was revolutionary but many practical experiments and calculations by other physicist have so far confirmed this theory.
However, there are still some things that we don’t understand about gravity. First we can’t understand how the universe can exert the gravitational force it does when to the visible eye and instrumentation there is considerably less mass. One possible explanation is dark matter. This is matter that would not be visible in any part of the electromagnetic spectrum. Scientist were able to use observations and calculations to help prove that this “invisible” matter exists.
Gravity is also important for scientists looking to find one big theory of everything or a unified field theory. This hypothesis basically claims that gravity, electromagnetic force, and the strong and weak nuclear forces are different aspects of the same force. Gravity is the missing piece as scientist try to fully comprehend what gravity is and why it works the way it does.
You can also check out Astronomy Cast. You should listen to episode 102 on gravity.